In automatic cut-sheet printers, a stack of paper, cut to uniformly sized sheets, is automatically fed to a printer, typically using a roller assembly or other mechanisms. An important function of the printer feed mechanism is to control the parallelism between the top edge of the sheet of paper and the first line of print contained thereon, i.e., the amount of skew between the paper and the print. Even a small amount of skew between the paper and the print will cause the printing to appear crooked. Larger amounts of skew may cause buckling of the paper, resulting in uneven print quality or jamming of the paper within the printer. The skew is generally induced when the paper is loaded into and/or picked from a stack of paper in a supply tray. Accordingly, it is desirable to minimize the amount of skew between the paper and the printing assembly once the paper has been picked and before it is printed on.
Prior art printing devices use a variety of techniques and apparatus to minimize skew. Some printers minimize skew by forcing a sheet of paper into a pair of stalled rollers, creating a buckle in the paper and forcing the leading edge of the paper to be parallel with the roller pair. The rollers are then activated to advance the paper into the print zone. Such a technique requires some type of clutching mechanism to stall the rollers long enough to allow the paper to be fed into the nip between the rollers. Further, this technique requires accurate control of the paper while it is buckling, as the buckle must be large enough to correct the skew, yet small enough that the paper does not flip out of the nip between the stalled rollers. Other prior art printers use tapered rollers which direct the sheet of paper against a reference wall, forcing it into alignment therewith and eliminating any skew before printing. This technique requires a large, flat surface in the area of the roller assembly and is relatively slow. Still other printers have no skew correction mechanism at all, relying entirely on the accurate feeding of paper into the roller assembly.
In addition to minimizing skew, the feed mechanism of a printer must maintain accurate control of each sheet, from the time it is picked from the stack until it is ejected from the printer. The paper feed mechanisms of typical prior art printers use separate motors and gear arrangements to pick the paper from a stack, deliver the paper to the printing assembly, line feed the paper and eject the paper once printed. Such feed mechanisms often encumber the carriage drive motor and have complex timing schemes requiring triggering devices, such as solenoids. The large number of motors and other electrical components increases the cost of the printer. Further, complex feed mechanisms increase the amount of time necessary to pass a page through the printer, as well as the chances of paper jams and skew errors.
Accordingly, it is desirable to control the feed of paper through a printer using a minimum number of control devices so as to reduce the cost of the printer and increase the printer reliability and throughput.
It is, therefore, an object of the present invention to provide a paper control apparatus in a printer which has relatively few components and particularly few active components, such as motors and solenoids.
It is a further object of the present invention to provide a paper control apparatus in a printer which minimizes the possibility of catching and paper jams.
It is another object of the present invention to provide a paper control apparatus in a printer which may be driven by a single motor.
It is another object of the present invention to provide a paper control apparatus in a printer which may be implemented economically.
It is a further object of the present invention to provide a paper control apparatus in a printer which increases the throughput of the printer.
It is yet a further object of the present invention to provide a method for controlling the parallelism between the top edge of a sheet of paper and the print contained thereon.
It is a further object of the present invention to provide an active skew correction apparatus which operates quickly.
It is yet another object of the present invention to provide an active skew correction apparatus which does not require special timing or active triggering mechanisms, such as motors or solenoids.
It is yet a further object of the present invention to provide an active skew correction apparatus which may be implemented economically.